Method of monitoring a vascular access and an apparatus for extracorporeal treatment of blood with a device for monitoring the vascular access

ABSTRACT

A method and device for monitoring a vascular access during a dialysis treatment in which the pressures in both the arterial and venous branches of the extracorporeal blood system are monitored by pressure sensors. Characteristic values for the condition of the vascular access are calculated in a computer unit from the pressures in the arterial and venous branches of the extracorporeal system, and these values are analyzed in an analyzer unit to detect a defective vascular access.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of monitoring avascular access during an extracorporeal blood treatment, in particulara chronic blood purification therapy such as hemodialysis,hemofiltration and hemodiafiltration and an apparatus for extracorporealblood treatment, in particular for hemodialysis, hemofiltration andhemodiafiltration with a device for monitoring the vascular access.

BACKGROUND OF THE INVENTION

[0002] With the known methods of chronic blood purification therapy suchas hemodialysis, hemofiltration and hemodiafiltration, the patient'sblood is passed through an extracorporeal system. Arteriovenousfistulas, vascular implants and/or various catheters are used as accessto the patient's vascular system. Typical flow rates in a vascularaccess are in the range of 1100 mL/min. The patient is usually connectedto the extracorporeal system by dialysis cannulas which are used to tapinto the fistula or the vascular implant.

[0003] If the connection between the extracorporeal system and thevascular system becomes undone or a blood leak occurs in theextracorporeal system, the patient can be prevented from bleeding todeath only if the extracorporeal blood flow is stopped within a fewminutes. Therefore, extracorporeal blood systems are generally equippedwith protective systems to constantly monitor the arterial and venouspressure (P_(art) and P_(ven)) within the system as well as theadmission of air into the extracorporeal system.

[0004] In the event of an alarm, the blood treatment is stopped, thevenous clamp is closed and acoustic and optical warning signals aretriggered. The protective system based on the pressure measurementresponds when the arterial or venous pressure in the extracorporealsystem changes by more than ±60 mm Hg. The alarm limits are selected sothat a change in the position of the patient does not trigger an alarm.

[0005] If the connection between the patient and machine becomes undoneat the arterial connection, i.e., at the cannula establishing thepatient's blood flow to the extracorporeal system, the pressure-basedprotective system on the machine responds rapidly, as explained below.The dialysis cannula presents the greatest flow resistance in theextracorporeal system. When air is drawn through the cannula into thearterial vacuum system of the extracorporeal system, the flow resistanceof the cannula drops by a factor of 10³ in proportion to the differencein density between the blood and air. Thus, the arterial vacuum in theextracorporeal system collapses suddenly.

[0006] However, the response of the pressure-based protective system isnot always guaranteed in the case when the venous cannula has becomedetached from the vascular access. On the venous side, the purifiedblood is supplied to the patient at an excess pressure, with the venousexcess pressure being proportional to the delivery rate of the bloodpump. This prevents penetration of air through the cannula into theextracorporeal system, which would be the case on the arterial vacuumside. Therefore, the flow resistance of the venous cannula does notchange, and the venous pressure on the machine end drops only by theamount of the pressure in the patient's vascular access. Thus, thechange in venous pressure in the extracorporeal system is usually toolow to trigger the pressure-based protective system to respond. Theadditional hydrostatic pressure difference between the venous pressuresensor and the cannula triggers a machine alarm only when the venouscannula is definitely below the fistula after slipping out of thevascular access.

[0007] In the case of a blood leak in the venous tubing system, it mayalso occur that the resulting venous pressure drop is not sufficient toguarantee that the existing pressure-based protective system will betriggered.

[0008] In addition to the above method, where the pressure in thearterial branch of the extracorporeal system is monitored to detectwhether the arterial cannula has slipped out, and where the pressure inthe venous branch of the extracorporeal system is monitoredindependently of the pressure monitoring in the arterial branch todetect whether the venous cannula has slipped out, there are knownmonitoring systems that monitor pressure pulses propagating in theextracorporeal system.

[0009] International Patent No. WO 97/10013 describes a dialysis machinehaving such a monitoring system which monitors the pressure pulses inthe venous blood line produced by the blood pump in the arterial bloodline.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide a method ofmonitoring a vascular access during an extracorporeal blood treatmentthat allows reliable detection of the venous cannula slipping out of thevascular access as well as reliable detection of a blood leak in thevenous branch of the extracorporeal system while also requiring aminimum of equipment. This object is achieved by a method of calculatingcertain characteristic values from the pressure in a patient'sextracorporeal blood path during treatment and comparing thesecharacteristic values to predetermined ranges for these values. Themethod includes monitoring a vascular access during an extracorporealblood treatment, wherein a patient's blood is provided from the vascularaccess to an extracorporeal blood path. The extracorporeal blood pathincludes an arterial line which is in fluid communication with anarterial access at one end and the inlet of a blood treatment device atthe other end. The blood path also includes a venous line extending fromand in fluid communication with an outlet in the blood treatment deviceand in fluid communication with a venous vascular access. According tothe method, the fluid pressure within the arterial and venous lines ismeasured and from these measured values characteristic values for theintegrity of the vascular access are calculated. The integrity of thevascular access is determined by comparing the characteristic values toranges of predetermined values.

[0011] Another object of the present invention is to create an apparatusfor extracorporeal blood treatment with a device for monitoring avascular access that detects whether the venous cannula has slipped outof the vascular access as well as whether there is any blood leak in thevenous branch of the extracorporeal system with a high reliability whilerequiring a minimum of additional equipment. This object is achieved bya system that will calculate certain characteristic values from thepressure in a patient's extracorporeal blood path during treatment andcompare the calculated characteristic values to predetermined ranges forthese values. The system according to the invention includes anextracorporeal blood path having an arterial line which is in fluidconnection with an arterial vascular access and an inlet to a bloodtreatment device. The system further includes a venous line which is influid connection with an outlet of the blood treatment device and avenous vascular access. The system also includes an arterial fluidpressure sensor connected to the arterial line that is adapted formeasuring pressure within the arterial line and a venous fluid pressuresensor connected to the venous line that is adapted for measuringpressure within the venous line. The system further includes a computerunit connected to the arterial and venous pressure sensors, which isadapted for generating characteristic values related to the integrity ofthe vascular access from measured arterial and venous pressures. Alsoprovided in the system according to the invention is an analyzer unitthat is adapted for comparing the calculated characteristic values topredetermined ranges values in order to assess the integrity of thevascular access.

[0012] The method according to the present invention can be designed asa protective system integrated into the machine, using sensors that arealready present in the known blood treatment machines. Thus, the onlyrequired change in the machine to implement the protective system ismodifying the machine control.

[0013] The method according to the present invention is based on thefact that both the pressure in the arterial branch and the pressure inthe venous branch of the extracorporeal system are monitored to detectwhen the venous cannula slips out of the vascular access or when thereis a blood leak in the venous branch of the extracorporeal system.Values characteristic of the condition of the vascular access arecalculated from the pressure in the arterial and venous branches of theextracorporeal system and are then analyzed to detect a defectivevascular access.

[0014] With the method according to the present invention, it ispossible to reliably detect not only whether the venous cannula hasslipped out or whether there is a blood leak in the venous branch of theextracorporeal system, but also whether the arterial cannula has slippedout and whether there is a blood leak in the arterial branch of theextracorporeal system.

[0015] The method according to the present invention can also becombined with other methods of detecting a defective vascular access.This further increases the reliability of the monitoring system.

[0016] For the case when the vascular access is defective, an acousticand/or a visual alarm is preferably triggered. In addition, the bloodflow in the extracorporeal system can be interrupted to prevent anyblood loss. The blood flow can be interrupted using a suitable apparatusfor extracorporeal blood treatment, such as by stopping the blood pumparranged in the extracorporeal system and/or closing a safety valve,such as a hose clamp arranged in the extracorporeal system.

[0017] Monitoring for a defective vascular access with the methodaccording to the present invention is possible not only in an apparatusfor hemodialysis, hemofiltration or hemodiafiltration, but also in cellseparators in which a donor's blood is centrifuged in an extracorporealsystem, thereby separating it into its components.

[0018] The method according to the present invention for monitoring avascular access and an apparatus for extracorporeal blood treatment witha device for monitoring the vascular access are explained in greaterdetail below with respect to the drawings and on the basis of apreferred embodiment.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 is a table showing the influences that can contribute to achange in the pressure in the arterial and venous branches of theextracorporeal system.

[0020]FIG. 2 depicts one embodiment of an apparatus for extracorporealblood treatment with a device for monitoring the vascular access in asimplified schematic diagram.

[0021]FIG. 3 is a flow chart depicting the monitoring device.

[0022]FIG. 4 shows arterial or venous pressure as a function of theblood flow in the extracorporeal system.

[0023]FIG. 5 depicts change in arterial and venous pressure in theextracorporeal system with a change in position of the patient.

[0024]FIG. 6 demonstrates pressure relationships in the extracorporealsystem when a venous cannula slips out of the vascular access.

[0025]FIG. 7 demonstrates pressure relationships when there is a bloodleak in the venous branch of an extracorporeal system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] During extracorporeal blood treatment, the arterial and venouspressures (P_(art) and P_(ven)) in the extracorporeal system aremeasured with a frequency f, and the sum P_(S) of the venous andarterial pressures is formed (equation 1).

P _(S) =P _(ven) +P _(art)  (equation 1)

[0027] P_(art)=measured arterial pressure in the extracorporeal system

[0028] P_(ven)=measured venous pressure in the extracorporeal system

[0029] In addition, the pressure difference ΔP between the venous andarterial pressures is also calculated:

delta P=P _(ven) −P _(art)  (equation 2)

[0030] The change in the sum of the arterial and venous pressures isgiven by:

d(P _(S))=P _(SN) −P _(SN−1)  (equation 3)

[0031] P_(SN)=instantaneous measured total pressure

[0032] P_(SN−1)=previous measured total pressure

[0033] The change in the pressure difference is given as follows:

d(delta P)=delta P _(N)−delta P _(N−1)  (equation 4)

[0034] delta P_(N)=instantaneous measured pressure difference

[0035] delta P_(N−1)=previous measured pressure difference

[0036] A defective vascular access is deduced when the followingconditions are met for N successive measurements:

[0037] a) ΣN d(P_(S)) is negative and is less than a threshold value M₁(equation 5)

[0038] b) ΣN d(ΔP) is negative and is less than a threshold value M₂(equation 6)

[0039] In this case, either the venous cannula has slipped out of thevascular access, or there is a blood leak in the venous branch of theextracorporeal system.

[0040] The table (FIG. 1) shows influences that can contribute toward achange in the venous and arterial pressure in the extracorporeal system.This table shows that in addition to detecting when the venous cannulaslips out, or whether there is a blood leak in the venous branch of theextracorporeal system, it is also possible to detect when the arterialcannula slips out or if there is a blood leak in the arterial branch ofthe extracorporeal system.

[0041] It can be deduced that the arterial cannula has slipped out orthat there is a blood leak in the arterial branch of the extracorporealsystem if the following conditions are met:

[0042] a) ΣN d(P_(S)) is positive and is greater than a threshold valueM₃ (equation 7)

[0043] b) ΣN d(ΔP) is positive and is greater than a threshold value M₄(equation 8)

[0044]FIG. 2 shows a simplified schematic diagram of a dialysis machinewith a device for monitoring the vascular access.

[0045] The dialysis machine has a dialyzer 1 which is subdivided by asemipermeable membrane 2 into a blood chamber 3 and a dialysis fluidchamber 4. An arterial blood line 5 is connected to the inlet of theblood chamber and to a peristaltic blood pump 6. Downstream from bloodchamber 3, a venous blood line 7 leads from the outlet of the bloodchamber to the patient. A drip chamber 8 is connected to the venousblood line 7. Cannulas 5 a, 7 a are inserted into the patient andconnected to the ends of the arterial and venous blood lines 5, 7. Thearterial and venous blood lines preferably are part of a tubing systemdesigned to be disposable.

[0046] Fresh dialysis fluid is prepared and located at a dialysis fluidsource 9. A dialysis fluid inlet line 10 leads from dialysis fluidsource 9 to the inlet of the dialysis fluid chamber 4 of dialyzer 1,while dialysis fluid outlet line 11 leads from the outlet of thedialysis fluid chamber to a drain 12.

[0047] The dialysis machine may also have other components, such as abalancing device and an ultrafiltration device, etc., which are notshown here for the sake of simplicity.

[0048] To interrupt the blood flow, a cutoff clamp 13 which is operatedelectromagnetically is provided on venous blood line 7 downstream fromdrip chamber 8. Arterial blood pump 6 and venous cutoff clamp 13 arecontrolled by a control unit 16 over control lines 14, 15.

[0049] Device 17 for monitoring the vascular access has an arterialpressure sensor 18 monitoring pressure in arterial blood line 5 and avenous pressure sensor 19 monitoring pressure in venous blood line 7.The measured values of pressure sensors 18, 19 are sent over data lines20, 21 to a computer unit 22 which calculates characteristic values forthe condition of the vascular access from the measured values. Interimresults obtained in the calculation can be stored in a memory unit 23connected by a data line 24 to computer unit 22. To analyze thecharacteristic values for the vascular access, the monitoring unit hasan analyzer unit 25 which is connected to the computer unit by a dataline 26. Analyzer unit 25 is connected by a control line 27 to an alarmunit 28, which is in turn connected by a control line 29 to control unit16.

[0050] The functioning of monitoring device 17 is described in detailbelow with reference to FIG. 3.

[0051] The number N of successive measured values to be used for theanalysis, the measurement frequency f and threshold values M₁ and M₂ aredefined first. These values can be stored in memory unit 23 or they maybe preset by the user (step 1).

[0052] The susceptibility of the protective system to faults, e.g., inshort-term artificial pressure fluctuations, is reduced by analysis of Nsuccessive measured values. The value for N is an integer and depends onthe frequency f with which the pressure values are determined. It can beadapted to the respective attenuation D of the pressure sensors. Thefollowing boundary condition applies to N:

N>f·D  (equation 9)

[0053] At a typical measurement frequency f of ⅓Hz, for example, and anattenuation of approximately 18 seconds, it should consequently holdthat N>6. This guarantees that the pressure drop is detected completely.

[0054] The negative threshold values M₁ and M₂ characterize thesensitivity of the protective system. In general, the smaller the valuesof M₁ and M₂, the higher the threshold at which a machine alarm istriggered. To guarantee a response of the system, however, the valuesfor M₁ and M₂ must be greater than the negative value of the venouspressure in the vascular access.

[0055] After the venous cannula slips out of the vascular access, thearterial fistula pressure drops slightly due to bleeding at the venouspuncture site. This effect can be taken into account through a suitablechoice of M₂, where the condition M₁<M₂ must be met. Suitable valuesinclude, for example, M₁=−15 mm Hg and M₂=−10 mm Hg.

[0056] The response time tau of the protective system is given by theequation:

tau=N/f  (equation 10)

[0057] During the dialysis treatment, the arterial and venous pressuresP_(art), P_(ven) are detected by pressure sensors 18, 19 at measurementfrequency f in N successive measurements (step 2).

[0058] After each measurement, computer unit 22 calculates the sum P_(S)of the venous and arterial pressures according to equation 1, and itcalculates the difference (delta P) between the venous and arterialpressures according to equation 2. These values are stored in memoryunit 23. Using equation 3, computer unit 22 calculates the difference,d(P_(S)), between the sum P_(SN) of the venous and arterial pressures ofa subsequent measurement and the sum P_(SN−1) of the venous and arterialpressures from a preceding measurement. Computer unit 22 calculates thedifference, d(delta P), between the difference d (delta P) between thearterial and venous pressures from a preceding measurement delta P_(N)and the difference delta P_(N−1) between the arterial and venouspressures of a following measurement according to equation 4. Thesemeasured values are also stored in memory unit 23. The changes in sums,d(P_(S)), of the N successive measurements are then added up by computerunit 22 according to equation 5 to form a first characteristic value W₁for the condition of the vascular access. The changes in difference,d(delta P), are added up in computer unit 22 according to equation 6 toform a second characteristic value W₂ for the vascular condition (step3).

[0059] Analyzer unit 25 checks the signs of the two characteristicvalues W₁, W₂. If either of the characteristic values W₁, W₂ isnegative, no alarm is triggered (step 4). However, if bothcharacteristic values W₁ and W₂ are negative, the characteristic valuesare then compared with the first and second threshold values M₁, M₂(step 5).

[0060] For the case when W₁ is smaller than M₁ and W₂ is also smallerthan M₂, analyzer unit 25 delivers an alarm signal to alarm unit 28.Alarm unit 28 preferably generates an acoustic and/or visual alarm andcontrols the control unit 16, which in turn stops blood pump 6 andcloses the cutoff element 13. This guarantees that the patient will notbe endangered in the event that venous cannula 7 a slips out or there isa leak in venous blood line 7.

[0061] Similarly, a defective vascular condition can also be inferredaccording to equations 7 and 8 if both the first characteristic value W₁and the second characteristic value W₂ are positive, and the firstcharacteristic value W₁ is greater than a predetermined threshold valueM₃ and the second characteristic value W₂ is greater than apredetermined threshold value M₄. In this case, the arterial cannula hasslipped out or there is a blood leak in the arterial branch of theextracorporeal system.

[0062] Computer unit 22 with memory unit 23 and analyzer unit 25 may beparts of a microcomputer, such as are often present in known dialysismachines.

[0063] If the dialysis machine has an ultrafiltration device, themonitoring device can be deactivated within the first one to two minutesafter the ultrafiltration device is turned on or off in order to preventa false alarm.

[0064] The characteristic principles which form the basis of the methodof monitoring the vascular access according to the present invention areexplained below.

[0065] The arterial and venous pressures P_(art) and P_(ven) measured inthe extracorporeal system are composed of the dynamic pressure in theextracorporeal system produced by the flow of the blood pump and thedynamic pressure in the patient's vascular access.

[0066] The dynamic pressure in the extracorporeal system is a functionof the extracorporeal blood flow and the sum of the flow resistances inthe extracorporeal system. Since the arterial and venous flow resistancediffer due to the difference in geometry of the components through whichthe flow passes, the pressure sum P_(S) is also a function of the bloodflow.

[0067] As a rule, the delivery rate Q_(B) of the blood pump is set at afixed value in the known blood purification methods. Thus, the sum ofthe flow resistances in the extracorporeal system is also constant at aconstant viscosity of the blood. An increase in viscosity, e.g., due toultrafiltration, leads to a reduction in the reduced arterial pressureand to an increase in the excess venous pressure in the long run.Consequently, the pressure response with a steady increase in viscosityis identical to a slight increase in blood flow.

[0068]FIG. 4 shows the extracorporeal pressures as a function of bloodflow Q_(B) at a constant viscosity (fistula flow Q_(F)=1255 mL/min,arterial fistula pressure P_(F art)=27 mm Hg, venous fistula pressureP_(F ven)=17 mm Hg).

[0069] The above measurements and all the following measurements wereperformed during a simulated dialysis treatment. The vascular access wasa segment of tubing with an inside diameter of 8 mm connected to a gearpump. To measure the flow within the vascular access, a Doppler flowmeter was used. The arterial and venous pressures within the vascularaccess (P_(Fistel)) were checked with two pressure manometers and wereadjusted with two hose clamps. Water was used in all the measurements(T=37 degrees C.). The cannulas used to puncture the vessels wereconnected to the dialysis machine by two conventional tubing systems.

[0070] The dynamic pressure in the patient's vascular access,hereinafter referred to as the fistula pressure, is also a function ofthe viscosity of the blood, the systemic blood pressure and the systemicvascular flow resistances. The fistula pressure is thus apatient-specific parameter and also depends on the type of vascularaccess, the viscosity of the blood and the vascular system supplyingblood to the vascular access. By analogy, with the dynamic pressure inthe extracorporeal system, a change in the fistula pressure, e.g., dueto fluctuations in blood pressure, an increase in viscosity or a changein position of the patient, leads to a change in the arterial and venouspressure values.

[0071]FIG. 5 shows the behavior of the extracorporeal pressures at aconstant effective blood flow rate Q_(B) when the patient's positionchanges by delta h=−33.5 cm relative to the extracorporeal pressuresensors (Q_(B)=300 mL/min, Q_(F)=1252 mL/min; pressure in the vascularaccess: P_(F art)=27 mm Hg, P_(F ven)=17 mm Hg). This is the case, forexample, when the patient lies down or lowers his recliner. This reducesthe arterial and venous pressure values by the amount of the hydrostaticpressure difference (approximately 0.78 mm Hg per cm of heightdifference between the pressure sensor and the fistula). Since thearterial and venous pressures in the extracorporeal system change by thesame value when the patient changes positions, the pressure differencedelta P remains constant. However, the sum of the pressures P_(S) dropsby two times the amount of the hydrostatic pressure difference.

[0072] This behavior is manifested (although in a diminished form) evenwith a pressure drop during the treatment. In this case, the arterialand venous pressures in the extracorporeal system also drop.

[0073]FIG. 6 shows the pressure relationships when the venous dialysiscannula slips out of the patient's fistula (Q_(B)=300 mL/min, Q_(F)=1254mL/min, P_(F art)=27 mm Hg, P_(F ven)=17 mm Hg). The pressure at thevenous sensor of the extracorporeal system (P_(ven)) drops by the amountof the venous fistula pressure within about 15-20 seconds. The delaytime is determined by the electronic attenuation of the extracorporealpressure signals. The arterial pressure value (P_(art)) is reduced onlyslightly within the first minutes, as explained below.

[0074] The blood loss which is caused by the open venous puncture siteand leads to the drop in fistula pressure can be estimated roughly onthe basis of the law of conservation of energy: $\begin{matrix}{{P \cdot V} = {\frac{1}{2}{M \cdot v^{2}}}} & \text{(Equation 11)}\end{matrix}$

[0075] where the following abbreviations are used:

[0076] P: pressure

[0077] V: volume

[0078] M: mass

[0079] v: velocity

[0080] With V=M/rho, it follows for the velocity of the blood streamfrom the open puncture site: $\begin{matrix}{v = \sqrt{\frac{2P}{\rho}}} & \text{(Equation 12)}\end{matrix}$

[0081] where:

[0082] rho=density

[0083] At a venous fistula pressure of 17 mm Hg (=22.6 mbar) and a blooddensity of 1.0506 g/cm³ (37 degrees C.), the velocity of the stream isapproximately 2.1 m/s. Under the assumption of a uniform opening of thepuncture site, the volume flow with a cylindrical stream profile isgiven by the equation:

Q=πr ² ·v  (Equation 13)

[0084] With a typical cannula diameter of 1.6, a value of approximately250 mL/min (=⅕of the fistula flow) is obtained. The open puncture siteswells in vivo, so the effective diameter of the hole will be smallerthan the diameter of the cannula. Thus, the stated value is to beregarded as an upper limit.

[0085] Because pressure and flow are proportional, the venous fistulapressure is reduced by 17/5=3.4 mm Hg (Hagen-Poiseuille law). Due to thereduced venous fistula pressure, the extracorporeal arterial pressuredrops by the same amount. However, since the change in arterial pressureis always smaller than the change in venous pressure, both P_(S) anddelta P are reduced.

[0086]FIG. 7 shows the pressure relationships when there is a blood leakin the venous tubing system. The venous tubing was punctured with aninjection cannula below the drip chamber (Q_(B)=300 mL/min, Q_(F)=1250mL/min, P_(F art)=27 mm Hg, P_(F ven)=17 mm Hg, leakage rate 50 mL/min).The pressure at the venous sensor drops by approximately 33 mm Hg withinapproximately 15-20 sec at a leakage rate of 50 mL/min. However, thereis no significant change in the arterial pressure. By analogy with FIG.6, both delta P and P_(S) drop. Consequently, the changes in totalsaccording to equations 6 and 7 are negative in both cases.

What is claimed is:
 1. Method of monitoring a vascular access during anextracorporeal blood treatment comprising the steps of: (a) providingblood from the vascular access of a subject through an extracorporealblood path comprising an arterial line in fluid communication with thevascular access, a blood treatment device, and a venous line in fluidcommunication with the vascular access; (b) measuring fluid pressurewithin the arterial and venous lines; and (c) calculating first andsecond characteristic values for the integrity of the vascular accessfrom the measured arterial and venous pressures; wherein the integrityof the vascular access is determined by comparing the first and secondcharacteristic values to first and second ranges of predeterminedvalues.
 2. The method of claim 1 , wherein step (c) further comprisesthe steps of: (i) calculating the first characteristic value bycalculating a sum of the measured pressure in the venous line and themeasured pressure in the arterial line according to the equation:P_(S)=P_(VEN)+P_(ART); and (ii) calculating the second characteristicvalue by calculating a difference between the measured pressure in thevenous line and the measured pressure in the arterial line according tothe equation: ΔP=P_(VEN)−P_(ART).
 4. The method of claim 3 , whereinsuccessive measurements of the arterial and venous pressures are taken.5. The method of claim 5 , wherein step (c) further comprises the stepsof: (i) calculating a change in the pressure sum for successivemeasurements by subtracting the sum for a first measurement from the sumfor a second measurement; and (ii) calculating a change in the pressuredifference between successive measurements by subtracting the differencefor a first measurement from the difference for a second measurement. 6.The method of claim 5 , wherein step (c) further comprises the steps of:(i) calculating a first characteristic value by adding the changes inthe sum for a predetermined number of successive pressure measurements;and (ii) calculating a second characteristic value by adding the changesin a predetermined number of calculated pressure differences.
 7. Themethod of claim 6 , further comprising the step of generating a saferesponse in reaction to characteristic values selected from the groupconsisting of: (i) a first characteristic value below the first range ofvalues, and a second characteristic value below the second range ofvalues, and (ii) a first characteristic value above the first range ofvalues, and a second characteristic value above the second range ofvalues.
 8. The method of claim 7 , wherein the safe response is an alarmselected from the group consisting of visual and audible alarms.
 9. Themethod of claim 7 , wherein the safe response comprises closing a cutoffclamp in the venous line.
 10. The method of claim 7 , wherein the saferesponse comprises interrupting the provision of blood to theextracorporeal blood path.
 11. The method of claim 7 , wherein the saferesponse comprises stopping a blood pump provided in the arterial line.12. A system for extracorporeal blood treatment comprising: (a) anextracorporeal blood path comprising an arterial line in fluidcommunication with a first vascular access and an inlet to a bloodtreatment device, and a venous line in fluid communication with anoutlet of the blood treatment device and with a second vascular access;(b) an arterial fluid pressure sensor in communication with the arterialline and constructed and arranged for measuring pressure within thearterial line; (c) a venous fluid pressure sensor in communication withthe venous line and constructed and arranged for measuring pressurewithin the venous line; (d) a computer unit connected to the arterialand venous pressure sensors, which is constructed and arranged forgenerating first and second characteristic values for the integrity ofthe vascular access from measured arterial and venous pressures; (e) ananalyzer unit constructed and arranged for analyzing the integrity ofthe vascular access by comparing the first and second characteristicvalues to first and second ranges of predetermined values.
 13. Thesystem of claim 12 , wherein generating the first characteristic valueincludes calculating a sum of the measured pressure in the venous lineand the measured pressure in the arterial line according to the equationP_(S)=P_(VEN)+P_(ART), and generating the second characteristic valueincludes calculating a difference between the measured pressure in thevenous line and the measured pressure in the arterial line according tothe equation ΔP=P_(VEN)−P_(ART).
 14. The system of claim 13 , furthercomprising a memory unit connected to the computer unit, which isconstructed and arranged for storing successive pressure sum andpressure difference values calculated by the computer unit andtransmitting the stored values to the computer unit.
 15. The system ofclaim 14 , wherein generating the first characteristic value furtherincludes calculating a change in the pressure sum for successivemeasurements by subtracting the sum for a first measurement from the sumfor a second measurement, and generating the second characteristic valuefurther includes calculating a change in the pressure difference betweensuccessive measurements by subtracting the difference for a firstmeasurement from the difference for a second measurement.
 16. The systemof claim 15 , wherein the first characteristic value is calculated byadding the changes in the sum of a predetermined number of successivemeasurements, and the second characteristic value is calculated byadding the changes in the predetermined number of calculated pressuredifferences.
 17. The system of claim 12 , wherein the analyzer unit isfurther constructed and arranged for generating a safe response inreaction to characteristic values selected from the group consisting of:(i) a first characteristic value below the first range of values, and asecond characteristic value below the second range of values, and (ii) afirst characteristic value above the first range of values, and a secondcharacteristic value above the second range of values.
 18. The system ofclaim 17 , wherein the smallest value in the first range ofpredetermined characteristic values (M₁) is less than the smallest valuein the second range of predetermined characteristic values (M₂).
 19. Thesystem of claim 18 , wherein M₁ is −15 mmHg and M₂ is −10 mmHg.
 20. Thesystem of claim 16 , wherein the predetermined number of successivemeasurements is greater than a product of the frequency of arterial andvenous pressure measurement and a value for the attenuation of thearterial and venous pressure sensors.
 21. The method of claim 17 ,wherein the safe response is an alarm selected from the group consistingof visual and audible alarms.
 22. The method of claim 17 , wherein thesafe response comprises closing a cutoff clamp in the venous line. 23.The method of claim 17 , wherein the safe response comprisesinterrupting the provision of blood to the extracorporeal blood path.24. The method of claim 17 , wherein the safe response comprisesstopping a blood pump provided in the arterial line.
 25. The system ofclaim 12 , wherein the system is incorporated into a dialysis machine.